In situations wherein a prescription lens user must also use a device having a plano protective lens, e.g. scuba mask, gas mask, respirator, it is desirable to mount the prescription lens in the protective device to provide an optical system. Conventional methods for mounting the prescription lenses include mounting the prescription lens behind or in front of the plano lens or by cementing the prescription lens to the plano lens.
Each of the conventional mounting methods have disadvantages.
A prescription lens mounted behind or in front of plano lens may become dislodged during use. If a prescription lens mounted behind the plano lens, i.e. between the plano lens and the wearers eye, becomes dislodged, the wearer must remove the protective device in order to remount the prescription lens. The prescription lens may cause discomfort or injury while dislodged by contacting the wearers facial features. If a prescription lens mounted in front of the plano lens becomes dislodged, the prescription lens may be lost.
Mounting a prescription lens in back of or in front of a plano lens juxtaposes a number of lens surfaces, which the wearer must look through. The optical performance of an optical system having a plurality of juxtaposed lens surfaces is compromised by reflections within the system. Performance may be improved by using expensive antireflective coatings.
Cementing the prescription lens to a plano lens requires a special prescription lens design whenever every lens in the series has the same back curve, i.e. a series of plus toric lenses. Such lenses are not standard in the ophthalmic industry and typically it would not be cost effective to stock a wide prescription range of such lenses for such a restricted purpose.